U.S. patent number 10,230,124 [Application Number 14/557,924] was granted by the patent office on 2019-03-12 for gravity induced flow cell.
This patent grant is currently assigned to Massachusetts Institute of Technology. The grantee listed for this patent is Massachusetts Institute of Technology. Invention is credited to W. Craig Carter, Xinwei Chen, Yet-Ming Chiang, Frank Yongzhen Fan, Ahmed Helal, Brandon James Hopkins, Zheng Li, Alexander H. Slocum, Kyle C. Smith.
United States Patent |
10,230,124 |
Hopkins , et al. |
March 12, 2019 |
Gravity induced flow cell
Abstract
The flow cell includes first and second reservoirs having a
selected volume containing a flowable redox electrode. A membrane
separates charged and discharged material. An energy-extraction
region includes electronically conductive porous current collectors
through or adjacent to which the flowable redox electrodes flow and
to which charge transfer occurs. Structure is provided for altering
orientation of the flow cell whereby gravity induces flow of the
flowable redox electrode between the first and second reservoirs to
deliver power. By varying the angle of the cell, flow rate and
power delivered on discharge or the charge rate on charge may be
varied.
Inventors: |
Hopkins; Brandon James
(Cambridge, MA), Slocum; Alexander H. (Bow, NH), Chen;
Xinwei (Cambridge, MA), Chiang; Yet-Ming (Weston,
MA), Fan; Frank Yongzhen (Cambridge, MA), Helal;
Ahmed (Cambridge, MA), Li; Zheng (Arlington, MA),
Smith; Kyle C. (Champaign, IL), Carter; W. Craig
(Jamaica Plain, MA) |
Applicant: |
Name |
City |
State |
Country |
Type |
Massachusetts Institute of Technology |
Cambridge |
MA |
US |
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Assignee: |
Massachusetts Institute of
Technology (Cambridge, MA)
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Family
ID: |
53266079 |
Appl.
No.: |
14/557,924 |
Filed: |
December 2, 2014 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20150155585 A1 |
Jun 4, 2015 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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61911101 |
Dec 3, 2013 |
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61912215 |
Dec 5, 2013 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M
8/20 (20130101); H01M 8/04276 (20130101); H01M
8/188 (20130101); Y02E 60/50 (20130101); Y02E
60/528 (20130101) |
Current International
Class: |
H01M
8/18 (20060101); H01M 8/20 (20060101); H01M
8/04276 (20160101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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WO 2012083239 |
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Jun 2012 |
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WO |
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2014/121276 |
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Aug 2014 |
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WO |
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Other References
"Notification of Transmittal of the International Search Report and
the Written Opinion of the International Searching Authority, or
the Declaration" for PCT/US2014/068342, dated Apr. 23, 2015. cited
by applicant .
Hopkins. Mechanical Design of flow batteries (Master Thesis). Mar.
16, 2013. Massachusetts Institute of Technology. cited by applicant
.
Fan et al. Polysulfide Flow Batteries Enabled by Percolating
Nanoscale Conductor Networks. Nano Letters. Apr. 9, 2014, pp.
2210-2218, vol. 14(4), American Chemical Society, Washington DC.
cited by applicant .
Medeiros et al. Magnesium-solution phase catholyte semi-fuel cell
for undersea vehicles. Journal of Power Sources, Oct. 1, 2004, pp.
226-231, vol. 136(2), Elsevier. cited by applicant .
Lei et al. An Alkaline Al--H2O2 Semi-Fuel Cell Based on a Nickel
Foam Supported Co3O4 Nanowire Arrays Cathode. Fuel Cells. Jun. 25,
2011, pp. 431-435, vol. 11(3), Wiley-VCH Verlag GmbH & Co KGaA.
Weinheim, Germany. cited by applicant .
Duduta et al. Semi-Solid Lithium Rechargeable Flow Battery.
Advanced Energy Materials, Jul. 20, 2011, pp. 511-516, vol. 1(4),
Wiley-VCH Verlag GmbH & Co. KGaA. Weinheim, Germany. cited by
applicant .
Li et al. Aqueous semi-solid flow cell: demonstration and analysis.
Physical Chemistry Chemical Physics. Jan. 1, 2013, pp. 15833-15839,
vol. 15(38). Royal Society of Chemistry. cited by applicant .
International Preliminary Report on Patentability dated Jun. 7,
2016 for PCT/US2014/068342. cited by applicant .
Leung et al., Progress in redox flow batteries, remaining
challenges and their applications in energy storage. RSC Adv.
2012;2:10125-56. cited by applicant.
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Primary Examiner: Fraser; Stewart A
Attorney, Agent or Firm: Wolf, Greenfield & Sacks,
P.C.
Government Interests
GOVERNMENT FUNDING
This invention was made with Government support under Grant Nos.
DE-AR0000065 and DE-AC02-06CH11357 awarded by the Department of
Energy. The government has certain rights in the invention.
Parent Case Text
This application claims priority to provisional application Ser.
No. 61/911,101 filed on Dec. 3, 2013 and to provisional application
Ser. No. 61/912,215 filed on Dec. 5, 2013, the contents of both
provisional applications of which are incorporated herein by
reference in their entirety.
Claims
What is claimed is:
1. Gravity induced flow cell comprising: first and second
reservoirs having a selected volume containing a flowable redox
electrode; a membrane separating charged and discharged material;
an energy-extraction region including electronically conductive
porous current collectors through or adjacent to which the flowable
redox electrode flows and to which charge transfer occurs; and
structure for altering orientation of the flow cell continuously
with respect to gravity in the range of about 0 to about 35 degrees
whereby gravity passively induces flow of the flowable redox
electrode along a flow direction between the first and second
reservoirs without pumps and valves, wherein the structure is
configured to change flow rate and power delivered on discharge or
charge rate on charge by varying the angle of the cell with respect
to gravity, wherein the first reservoir is disposed on a first end
of the energy extraction region, the second reservoir is disposed
on a second end, opposite the first end, of the energy extraction
region, the first end and the second end defining an axis along the
flow direction.
2. The flow cell of claim 1 further including a motor to vary angle
of the cell.
3. The flow cell of claim 1 wherein the energy-extraction region
has a volume and the ratio of the volume of the energy-extraction
region to reservoir volume is selected to be in the range of about
1 to about 100.
4. The flow cell of claim 1 further including structure for
providing pneumatic pressure to alter flow rate.
5. The flow cell of claim 1 further including valves to modulate
flow rates.
6. The flow cell of claim 1 wherein the electronically conductive
porous current collectors comprise carbon.
7. The flow cell of claim 6 wherein the carbon is selected from the
group consisting of glassy carbon, disordered carbon, and
graphite.
8. The flow cell of claim 6 wherein the carbon is in the form of
compacted fibers, woven fibers, paper or 3D reticulated foam.
9. The flow cell of claim 1 wherein the electronically conductive
porous current collectors are a metal or metal alloy.
10. The flow cell of claim 9 wherein the metal is selected from the
group consisting of aluminum, copper, nickel and stainless
steel.
11. The flow cell of claim 1 wherein the electronically conductive
porous current collectors are coated with a metal or metal
alloy.
12. The flow cell of claim 1 wherein the energy-extraction region
comprises current collector plates.
13. The flow cell of claim 1 wherein the flowable redox electrode
is a suspension.
14. The flow cell of claim 13 wherein the suspension includes
conductor particles.
15. The flow cell of claim 1 wherein the reservoirs and
energy-extraction region include a slippery, low friction or
non-wetting surface.
16. The flow cell of claim 1 wherein the flowable redox electrode
is a Li-polysulfide suspension.
17. The flow cell of claim 1 having a flowable redox electrode
working ion that is an alkali ion selected from the group
consisting of lithium, sodium, potassium and cesium.
18. The flow cell of claim 1 having a flowable redox electrode
working ion that is a divalent ion of magnesium or calcium.
19. The flow cell of claim 1 having a flowable redox electrode
working ion that is a trivalent ion of aluminum or yttrium.
Description
BACKGROUND OF THE INVENTION
This invention relates to flow cells and more particularly to a
flow cell that uses gravity to flow a redox electrode from one
reservoir or tank to a second reservoir or tank to deliver
power.
Flow batteries are known. Conventional flow batteries are often
very large and include multiple pumps, valves and sensors operated
by complex control systems. Such prior art flow batteries are often
leaky.
FIG. 1 is an illustration of a conventional flow battery
architecture. In FIG. 1, a cathode tank 10 and an anode tank 12
provide flowable electrodes through current collectors 14 and 16
separated by a membrane 18. Pumps 20 and 22 circulate the
respective flow electrodes. This arrangement is not optimal for
electronically conductive anode and cathode material.
An object of the present invention is a gravity induced flow cell
that requires no pumps and no valves and is passively driven by
gravity. It is also an object of the invention to provide a gravity
induced flow cell that may be manufactured using simple low cost
parts and methods, including but not limited to stackable
injection-molded plastic parts.
SUMMARY OF THE INVENTION
The gravity induced flow cell according to the invention includes
first and second reservoirs having a selected volume containing a
flowable redox electrode. A membrane is provided separating charged
and discharged material. The flow cell includes an
energy-extraction region including electronically conductive
current collectors through or adjacent to which the flowable redox
electrodes flow and to which charge transfer occurs. Optionally,
the current collector is a plate, a plate containing channels to
direct flow and/or increase surface area, a porous stationary
electronically conductive material, or a percolating network of
conductor particles or fibers that flows with the electrode.
Structure is provided for altering orientation of the flow cell
with respect to gravity whereby gravity induces flow of the
flowable redox electrodes between the first and second reservoirs.
In a preferred embodiment, by varying the angle of the cell, flow
rate and power delivered on discharge or the charge rate on charge
may be varied. It is preferred that the cell include a motor to
vary the angle of the cell with respect to gravity.
In a preferred embodiment, the energy extraction region has a
volume and ratio of the volume of the energy-extraction region to
reservoir volume is selected to be in the range of about 1 to about
1000. Pneumatic pressure may be added to alter flow rate in
addition to that induced by gravity. Valves may be included if
desired to modulate flow rates.
In another preferred embodiment, a stationary current collector
includes carbon. The carbon may be selected from the group
consisting of glassy carbon, disordered carbon, graphite, and
nanoparticulate carbon including fullerenes, carbon nanofibers and
carbon nanotubes, graphene, and graphene oxide. The carbon may be
in the form of a carbon plate, plate with nonplanar surface
features including channels, compacted fibers, woven fibers, paper
or 3D reticulated foam. A stationary current collector may be a
carbon coating on a support or substrate comprising an insulating
or conductive material.
In another preferred embodiment, the stationary current collector
is a metal or metal alloy such as aluminum, copper, nickel and
stainless steel. The metal or metal alloy may be in the form of a
metal plate, plate with non-planar surface features including
channels, compacted metal fibers, woven metal fibers, or 3D
reticulated metal foam. A stationary current collector may be a
metal or metal alloy coating on a support or substrate comprising
an insulating or conductive material.
In another preferred embodiment the stationary current collector is
a metal oxide, preferably an electronically conductive metal oxide
such as indium-tin-oxide (ITO), titanium oxide with a
oxygen/titanium atomic ratio less than 2, vanadium oxide with
oxygen/vanadium atomic ratio less than about 2.5, ruthenium oxide,
a transition metal oxide, a perovskite oxide including but not
limited to (La,Sr)MnO.sub.3, a spinel oxide including but not
limited to spinels containing the transition metals Fe, Co, Mn and
Ni, and mixtures and doped variants of such oxides including those
doped to impart n-type or p-type electronic conductivity. The metal
oxide may be in the form of a metal oxide plate, plate with
nonplanar surface features including channels, metal fibers, or
porous sintered metal oxide. A stationary current collector may be
a metal oxide coating on a support or substrate comprising an
insulating or conductive material.
In a particularly preferred embodiment, the flowable redox
electrode is a suspension and the suspension may include conductor
particles as well as active material particles. Due to the
existence of a percolating electronically conductive network in
such suspensions, the percolating network itself acts as an
extended, mobile current collector allowing for the electrochemical
reaction throughout the volume of the flow electrode. Such active
materials suspensions have been described in U.S. Pat. No.
8,722,227B2. Another preferred flowable redox electrode is metal
sulfide composition such as described in PCT/US2014/014681. The
contents of both of these references are incorporated herein by
reference. The flowable redox electrode working ion is an alkaline
ion selected from the group consisting of Li.sup.+, Na.sup.+,
K.sup.+ and Cs.sup.+. The working ion may also be a divalent ion of
magnesium or calcium. The working ion may also be a trivalent ion
of aluminum or yttrium. It is also preferred that the reservoirs
and energy-extraction region include a slippery, low friction or
non-wetting surface.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a schematic illustration of a prior art flow battery.
FIG. 2 is a perspective view of an embodiment of the gravity
induced flow cell of the invention.
FIGS. 3a and 3b are cross-sectional views of an embodiment of the
gravity induced flow cell disclosed herein.
FIG. 4 is an exploded view of an embodiment of the invention
disclosed herein.
FIG. 5 is a graph of current versus time for an embodiment of the
invention disclosed herein.
FIG. 6 a schematic illustration of another embodiment of the
invention disclosed herein.
FIG. 7 is a schematic illustration of another embodiment of the
invention disclosed herein.
FIG. 8 is a schematic illustration of a flow battery in which the
redox solutions contain nanoscale conductor networks forming an
infinite current collector.
FIGS. 9a and 9b is a schematic illustration of a flow battery
operating at .theta..degree. to induce flow by gravity with various
proposed system for controlling the gas flow rate, which
consequently controls the flow rate for the flowable electrode.
FIG. 9a a control valve system is placed in the gas flow channel to
control the gas flow rate. FIG. 9b the control valve can be
replaced by a porous media with known permeability.
FIGS. 10a and 10b depicts the predicted flow time of 3.75 aliquots
passing through the current collector region at different angle of
operation with respect to the horizontal based on our theoretical
model of GIF Cell. The model assumed no contact line pinning or
flow instability. FIG. 10a comparison between a surface with
stainless steel and Teflon. FIG. 10b comparison with various
permeability. The parameter a is defined to be
.alpha.=L.sub..mu./k.sub.a where L is the thickness of the membrane
(m), .mu. is the viscosity of the gas (Pas), k is the permeability
of porous membrane (m.sup.2) and A is the cross-sectional area of
the membrane (m.sup.2).
DESCRIPTION OF THE PREFERRED EMBODIMENT
With reference now to FIGS. 2, 3 and 4, a gravity induced flow cell
30 includes storage tanks 32 and 34. A membrane and current
collector region is shown at 36. The angle that the flow cell 30
makes with respect to gravity is adjusted by means of the structure
38 that may also include an electric motor for varying the angle.
FIG. 3b illustrates the gravity induced flow cell 30 positioned at
an angle .theta..
FIG. 4 is an exploded view of the flow cell 30. It is preferred
that the material that houses the suspension is selected to be
electrochemically compatible with the liquid electrolyte used. For
nonaqeuous electrolytes, and electrolytes comprising
alkyl-carbonates in particular, a suitable material is an ABS-like
plastic (Watershed XC 11122, DSM Somos).
FIG. 5 present results of a prototype embodiment according to the
invention. For this experiment, potentiostatic discharge was at
2.1V (to avoid precipitation of insoluble species). The flowable
electrode used was 2.5MS (as in Li.sub.2S.sub.8) in TEGDME (1 wt %
Li No.sub.3 with 0.5M Li TFSI) with 0.5 volume percent Ketjen
black. The graph of FIG. 5 shows that a higher flow rate means more
current. One can also see that current increases as the suspension
enters a flow channel.
FIGS. 6 and 7 are additional embodiments of the invention.
Additional ports in FIG. 6 are added to achieve greater uniformity
in flow. The addition of multiple channels slow the central region
of flow. The embodiment in FIG. 7 adds more ports and channels to
achieve greater uniformity in flow. The ends of the channels are
isolated to avoid bubble formation.
FIG. 8 illustrates an embodiment of the invention in which the
redox solutions contain nanoscale conductor networks.
The state-of-charge of the cell 30 disclosed herein, corresponding
to the working ion concentration in the negative and positive
electrodes, may be changed in one or more flow passes of the
electrode. When multiple passes are used to charge or discharge,
the cell is inverted for each pass. The energy dissipated is
limited primarily to the energy required to rotate and invert the
cell.
In a preferred embodiment, the ratio of the internal volume of the
energy-extraction region, also referred to as the "stack," to the
volume of the reservoir varies from about 1 to about 100.
Accordingly, the total energy and total charge/discharge time of
the battery varies. For example, when the stack is operated at a 1C
current rate, corresponding to the complete charge or complete
discharge of the material within the stack in one hour, flowing the
electrode at the rate of one stack volume per hour results in the
discharge of the batteries' stored energy in about 10 hours. The
gravity induced cell disclosed herein is particularly well suited
to the use of high energy density flow electrodes as the ratio of
stack volume to tank volume is generally higher, and the total
system size smaller, for a given stored energy. Thus, the size of
the unit that must be inverted is smaller and the dissipated energy
for inversion is lower.
Pneumatic pressure may be used in addition to gravity. The flow
rates may be modulated using valves located in the stack or tank or
between the stack and tank.
In one embodiment, the stack contains an electronically conductive
current collector through or adjacent, to which the flow electrode
flows, and to which charge transfer occurs.
In some embodiments of the invention disclosed herein the current
collector comprises carbon. The carbon may be selected from the
group consisting of glassy carbon, disordered carbon, graphite, and
nanoparticulate carbon including fullerenes, carbon nanofibers and
carbon nanotubes, graphene, and graphene oxide. The carbon may be
in the form of a carbon plate, plate with nonplanar surface
features including channels, compacted fibers, woven fibers, paper
or 3D reticulated foam. A stationary current collector may be a
carbon coating on a support or substrate comprising an insulating
or conductive material.
In another preferred embodiment the current collector is a metal or
metal alloy such as aluminum, copper, nickel and stainless steel.
The metal or metal alloy may be in the form of a metal plate, plate
with nonplanar surface features including channels, compacted metal
fibers, woven metal fibers, 3D reticulated metal foam. A stationary
current collector may be a metal or metal alloy coating on a
support or substrate comprising an insulating or conductive
material.
In another preferred embodiment the current collector is a metal
oxide, preferably an electronically conductive metal oxide such as
indium-tin-oxide (ITO), titanium oxide with a oxygen/titanium
atomic ratio less than 2, vanadium oxide with oxygen/vanadium
atomic ratio less than about 2.5, ruthenium oxide, a transition
metal oxide, a perovskite oxide including but not limited to
(La,Sr)MnO.sub.3, a spinel oxide including but not limited to
spinels containing the transition metals Fe, Co, Mn and Ni, and
mixtures and doped variants of such oxides including those doped to
impart n-type or p-type electronic conductivity. The metal oxide
may be in the form of a metal oxide plate, plate with nonplanar
surface features including channels, metal fibers, or porous
sintered metal oxide. A stationary current collector may be a metal
oxide coating on a support or substrate comprising an insulating or
conductive material.
In a particularly preferred embodiment, the flowable redox
electrode is a suspension and the suspension may include conductor
particles as well as active material particles. Due to the
existence of a percolating electronically conductive network in
such suspensions, the percolating network itself acts as an
extended, mobile current collector allowing electrochemical
reaction throughout the volume of the flow electrode. Such active
materials suspensions have been described in U.S. Pat. No.
8,722,227 B2. Another preferred flowable redox electrode is metal
sulfide composition such as described in PCT/US2014/014681. The
flowable redox electrode working ion is an alkaline ion selected
from the group consisting of Li.sup.+, Na.sup.+, K.sup.+ and
Cs.sup.+. The working ion may also be a divalent ion of magnesium
or calcium. The working ion may also be a trivalent ion of aluminum
or yttrium. It is also preferred that the reservoirs and
energy-extraction region include a slippery, low friction or
non-wetting surface.
The flow electrode may comprise water as a solvent. The flow
electrode may also be nonaqueous. In a particularly preferred
embodiment, the flow electrode is a suspension that may include
conductor particles. The suspension may include an electronically
percolating network, which may comprise any of the solids mentioned
including carbons, metal oxids, and metals and metal alloys. The
suspension may be electronically conductive or a mixed
electronic-ionic conductor.
In some embodiments of the invention the flow cell includes a flow
positive electrode or a flow negative electrode. The flow battery
may include a flow positive electrode and a flow negative
electrode. It is preferred that the materials of the reservoir end
of the stack are slippery, low friction materials that may have a
nonwetting surface. In some embodiments, the contact angle of the
flow electrode on a surface is greater than 45.degree. or greater
than 90.degree.. It is preferred that a non-stick surface be used
for the walls that are in contact with the flow electrodes. Such a
surface promotes plug-flow characteristic for the fluid, and
minimizes the residue in the flow channels during operation and
maximizes energy efficiency. Such a surface also promotes uniform
flow of the redox electrode in the channels of the stack.
A particularly preferred embodiment of the invention uses a
Li-polysulfide suspension as the flow electrode. Such a flow
electrode material optionally may include electronically conductive
particles that form an electronically percolating network rendering
the flow electrode electronically conductive. Because the
suspension fluid comprises an ether-based solvent, a coating of
thermoplastic or inorganic material insoluble in the solvent is
desired. Suitable candidates include polytetrafluoroethylene
(PTFE), fluorinated ethylene propylene, and boron nitride.
In one embodiment of the cell disclosed herein, the material that
houses the suspension fluid is an ABS-like plastic that has a glass
transition temperature of 39-46.degree. C. and softens above
50.degree. C. The surface of this material does not offer a
non-stick property to the suspension fluid and a non-stick coaling
is therefore desired. Traditional methods of coating this material
using a PTFE solution requires a curing temperature of 300.degree.
C. to obtain a non-porous coating that adheres to the wall.
Alternatively, a non-stick material composing a thin film may be
applied to the interior walls of the cell. In one example, a 0.02
cm thick film of PTFE with acrylic adhesive on one side is used.
The walls of the cell are cleaned with ethanol and the PTFE film is
carefully applied to the walls of the cell to prevent the formation
of air pockets.
In one embodiment, the walls of the cell are cleaned with ethanol
and a PTFE film is carefully applied to the walls to prevent
formation of air pockets.
In another embodiment, the surface of the tanks or stack are
rendered non-stick using a porous or nanoporous layer into which is
infused a liquid that is immiscible with the liquid of the flow
electrode.
As shown in FIGS. 9a and 9b the design of the GIF cell can include
a gas flow channel 40 which can be described as a pneumatic system.
The gas flow of GIF cell 30 can be controlled by a flow control
system, similar to any pneumatic system, which offers a resistance
to the flow of air thus generating a pressure drop as the air flows
(FIG. 9a). One embodiment of a flow control system is a control
valve 42. The control valve 42 can be in the form of a gate valve,
globe valve, pinch valve, diaphragm valve and needle valve. The
valve can be operated manually or by power actuator. Another
embodiment of a gas flow control system is the use of a permeable
porous system such as a membrane system 44 or a cylinder of a
porous media such as foam or filter (FIG. 9b). A porous system is
primarily a porous layer of media with known permeability. The
material for the porous system can be either organic (polymer) or
inorganic (ceramic). The permeability of the system can be achieved
by the following methods: (1) using another porous media/membrane
with different permeability, (2) putting the porous media/membrane
with same permeability in series, of (3) changing the effective
area of the gas passing through the porous media/membrane.
The use of a slippery surface affects the flow profile of the
suspension at different angles in a given flow channel (FIG. 10a).
Here, the term "aliquot" refers to the volume of the
electrochemically active "stack" in between the "tanks." A slippery
surface can be a surface of low surface energy such as Teflon.
Another embodiment is also a Liquid Infused Surface where a liquid
immiscible with the suspension is infused in a thin porous layer.
Another embodiment is the use of superhydrophobic, oleophobic
and/or omniphobic surfaces that do not stick to the suspension. For
instance, having a high-slip surface (Teflon) allows the
yield-stress suspension (0.5 vol % loading of carbon black in 2.5 M
of lithium polysulfide) to start flowing at a lower angle (ca.
5.degree.) compared to a stainless steel surface (FIG. 10a). The
flow rate is also less sensitive to the change of angle in the
region from 6.degree. to 12.degree. for the Teflon surface, which
enables a better controllability of the flow for the flow cell. The
flow profile of the suspension is dependent on the gas flow rate in
the flow cell (FIG. 10b) according to the model. In this example,
the gas flow rate can be tuned by .alpha., a parameter representing
the flow resistance of the flow control system and defined to
be
.alpha..DELTA..times..times..times..times..mu. ##EQU00001## where L
is the thickness of the membrane (m), .mu. is the viscosity of the
gas (Pas), k is the permeability of porous membrane (m.sup.2) and A
is the cross-sectional area of the membrane (m.sup.2). In general,
we can define a parameter .alpha. that represents the flow
resistance of the flow control system chosen that can be designed
by the strategies discussed in the earlier paragraph. For
example,
.alpha..DELTA..times..times. ##EQU00002## for systems where air
flow is linearly related to pressure such as membranes, porous
media and viscous dominated flow in channels; or,
.alpha..DELTA..times..times. ##EQU00003## where n>1 for inertia
dominated flows and for more complex devices such as control
valves.
In some preferred embodiments, the working ion of the battery is an
alkali ion including, but not limited to, Li.sup.+, Na.sup.+,
K.sup.+, Cs.sup.+. The working ion may be a divalent ion of
magnesium or calcium. The working ion may also be a trivalent ion.
In some embodiments the working ion is a trivalent ion including,
but not limited to, aluminum or yttrium.
In some embodiments, the flow electrode is a suspension comprising
a redox-active solution or suspension and a percolating network of
conductive particles. The flow electrode may comprise a suspension
of solid ion storage material particles including compounds that
store ions by intercalation, by alloy, or by carrying out a
conversion or displacement reaction. The flow electrode may be a
metal-sulfide solution or suspension. In some embodiments, the
metal sulfide system is lithium-sulfide, sodium-sulfide, or
magnesium-sulfide. The gravity induced flow cell of the invention
may be an energy storage device that is a flow capacitor. The
energy storage device may be an electrolytic or electrochemical
flow capacitor. The flow capacitor may include an aqueous or
non-aqueous solvent. One or more of the flow electrodes of such a
flow capacitor includes a suspension of carbon particles.
The gravity induced flow cell may be a hybrid device in which
Faradaic as well as capacitive reactions take place, is a preferred
embodiment, one of the electrodes stores charge through a Faradaic
reaction and another electrode stores charge through capacitive
storage. The capacitor electrode stores charge by absorption of
charge or formation of an electrical double layer. In some
embodiments, the capacitor electrode comprises particles of less
than 10 micrometers, less than 1 micrometer, less than 0.1
micrometer, less than 0.01 micrometer, or less than 0.001
micrometer average particle diameter. In some embodiments of the
invention, the flow electrode of the flow battery or flow capacitor
is electronically conductive, having an electronic conductivity of
at least 10.sup.-6 S/cm, at least 10.sup.-5 S/cm, at least
10.sup.-4 S/cm, at least 10.sup.-3 S/cm, at least 10.sup.-2 S/cm,
at least 10.sup.-1 S/cm an or at least 1 S/cm.
It is recognized that modifications and variations of the invention
will be apparent to those of ordinary skill in the art and it is
intended that all suck modification and variations be included
within the scope of the appended claims.
* * * * *